The present invention relates to semiconductor substrates and more specifically to a process for improving high frequency isolation between structures on a monolithic microwave integrated circuit.
Radio frequency (xe2x80x9cRFIxe2x80x9d) and microwave integrated semiconductor circuits are used extensively in wireless communications. As technology progresses, these semiconductors must operate at higher frequencies, provide better performance, and permit increased miniaturization. One performance parameter whose improvement carries significant benefit toward achieving increased performance goals is RF isolation. RF isolation refers to the degree with which an RF signal present on a Monolithic Microwave Integrated Circuit (xe2x80x9cMMICxe2x80x9d) does not couple into a different RF signal path and present itself as noise. RF coupling can occur in various ways such as RF noise traveling through conductive traces that carry DC bias voltages or reference potential and through vias used to ground two or more signal lines. RF isolation becomes more of a a challenge at higher frequencies and at higher signal power levels.
There are many conventional design methods used to maximize RF isolation including spacing conductive traces sufficiently far apart from each other to minimize the coupling of electric fields from one trace into a neighboring trace and coupling capacitors connected to DC lines that appear as an RF short and a DC open circuit to short out any RF energy that couples into the bias lines. The additional space and discrete components that are added to the circuit increase the size of the MMIC which is in contravention to the interest in miniaturization.
There is a need, therefore, for an apparatus that improves RF isolation in MMICs without increasing the size of the circuit.
It is an object of an embodiment according to the teachings of the present invention to improve the RF isolation on an MMIC.
It is an object of an embodiment according to the teachings of the present invention to improve the RF isolation without increasing the size of the MMIC or requiring additional circuitry.
An isolation structure comprises a semi-insulating substrate with an active region on a surface of the substrate. The active region is surrounded by a high resistivity region on the surface of said substrate, and a conductive material is disposed over a portion of the active region and a portion of the high resistivity region.
A process for creating a microwave semiconductor isolation structure on a substrate comprises the steps of creating an active layer on a surface of the substrate having a lower sheet resistivity than bare substrate and masking portions of said active layer with a photoresist material. The process includes the further steps of damaging portions of the active layer that are not protected by said photoresist layer to create localized regions of a high resistivity layer adjacent localized regions of an active layer. After removing the photoresist the process further includes forming a conductive material having first and second contiguous portions, the first portion disposed over a portion of the active region and the second portion disposed over a portion of the high resistivity region.
It is a feature of an embodiment according to the teachings of the present invention that a conductive material is disposed over both active and resistive layers on the substrate.
It is an advantage of an embodiment according to the teachings of the present invention that RF isolation is improved for RF vias and bias traces in an MMIC without increasing the size of the circuit by providing a weaker coupling path relative to the path to ground.
It is a further advantage of an embodiment according to the teachings of the present invention that bias traces may be more closely spaced than in prior art devices.
It is a further advantage of an embodiment according to the teachings of the present invention that a more than one RF signal line may be terminated to a single via by providing a higher relative coupling impedance between the RF paths than the impedance of the path to ground.